The invention relates to hydrogen phosphide generating pest control agents on the basis of alkaline earth or earth metal phosphides comprising a coating for retarding the onset of the generation of hydrogen phosphide therefrom.
Pest control agents in solid form are known which when exposed to air slowly evolve gaseous components. Such pest control agents are adapted to generate hydrogen phosphide from hydrolysable alkaline earth and/or earth metal phosphides, in particular on the basis of aluminium phosphide and magnesium phosphide, when acted upon by the moisture content of air or stored commodities. Such pest control agents are employed for example for combating pests such as e.g. bugs, worms, cockroaches or other insects and also rodents, e.g. mice and rats, e.g. in stores, mills, shipholds, railway carriages, trucks, containers, storage sheds, plastic lined earth silos and dumps or silos for grain, legumes, nuts, cocoa beans, tobacco or other foodstuffs or luxury goods, animal feeds and other processed or unprocessed commodities and various environments whether used for storage or containing such commodities or not.
Such pest control agents have in the past 10 to 20 years gained the upper hand to an increasing extent over other fumigating agents such as methyl bromide, ethylene dibromide and ethylene oxide. Reasons for this are on the one hand the favorable properties of hydrogen phosphide; i.e., it in no way impairs the quality of the fumigated commodities, and it rapidly penetrates into the interior, e.g. in the case of grain into the seeds, where it destroys animal organisms including all their development stages. However, it is dissipated just as rapidly again by aeration which follows the fumigation. On the other hand the application is relatively simple because the products, e.g. in the form of pressed bodies such as tablets and pellets can either be admixed to the flow of grain or be spread out on the floor of storage halls. However, such pest control agents can also be filled as compositions in powder form into sachets of special paper or suitable non-woven fabrics (fleeces) and can in that form be introduced into the stored commodities. The pest control agent releases the hydrogen phosphides formed by hydrolysis through the walls of the sachets to the outside. A further development of this method of application for the fumigation process involves the use of a belt comprising a multitude of pockets for the direct accommodation of the pest control agent or of sachets thereof.
Although, in contrast to the use of other gases, it is usually not necessary to wear gas-masks when employing hydrogen phosphide generating pest control agents, situations do arise when measurable hydrogen phosphide concentrations occur even during the application stage--in particular when the temperatures and moisture are relatively high.
In the light of ever-increasing environmental consciousness, this fact gives rise to increasing criticism, of the use of such pest control agents because hydrogen phosphides is highly toxic also for humans and higher animals.
The result has been that in some countries or in part of such countries labor organisations are prohibiting the application of hydrogen phosphide developing pest control agents by their members, not the least because, in the past several accidents with fatal consequences occurred.
In order to mitigate the high moisture sensitivity of the abovementioned preparations during handling, packaging and application, attempts had previously been made (DE-GM 1829597) to coat the phosphide particles or the entire tablets with hard paraffin, natural or synthetic resins, waxes or silicones. For that purpose the tablets which had sharp corners and very rough surface configurations were dipped into solutions of the coating substance. The preferred embodiments provided for the incorporation of decomposable substances, e.g. ammonium carbamate for bursting open the coating. These coatings, which, if complete, were completely impervious to moisture, were unsuccessful. If the coating were thin, the cover was incomplete and the tablets began to release gas immediately on exposure to moist air. Such coatings also offered no effective protection against violent reaction with liquid water. On the other hand, if the coating was relatively thick, it prevented the access of moisture to the phosphide entirely. Such tablets then did not release phosphine at all unless the coating was broken by mechanical action. From those unsuccessful attempts a further proposal arose (U.S.-PS No. 3,132,067) to completely envelop the individual phosphide particles with a coating, in particular of hard paraffin wax. In order to permit access to the phosphide particles of ambient moisture when using the preparation, it was considered necessary to incorporate a bursting agent such as ammonium carbamate. In this regard, see U.S. Pat. No. 4,347,241, which discusses the use of a bursting agent. These coatings of moisture-impervious material, in combination with the ammonium carbamate, although intended to delay the onset of phosphine generation, did not achieve this in practice. If the mass loss of the tablets due to the loss of carbamate was taken into account it was found that these tablets in fact commenced releasing a substantial amount of phosphine within minutes of exposure to humidity and continued doing so for several days, albeit at a reduced rate and with a reduced risk of autoignition.
It was also proposed to admix to the phosphide with a very slowly evaporating substance such as p-dichlorobenzene, camphor, methaldehyde and paraformaldehyde and to press the mixture into tablets (DP 1 155 631) in order to attain a retarded gas evolution. Quite apart from the fact that, because of the absence of a dense coating, the phosphide particles present on the outside of the tablets commence immediately to emit gas and therefore the aforesaid deficiencies of the prior art compositions are not eliminated, the substances proposed have all been toxic. Thus, methaldehyde for example, is employed as a poison for controlling snails. Such substances are therefore quite unsuitable for the treatment of foodstuffs such as grain, nuts or cocoa beans.
Prior to the present invention all attempts had failed to produce a metal phosphide preparation, e.g., in the form of a tablet, a tablet with a substantial "built-in" predetermined initial delay period preceding the onset of the desired phosphine generation at a normal rate--in fact, for more than 20 years the attainment of this desirable effect had been considered impossible.